1. Technical Field
The present invention relates generally to wireless communications; and more particularly to Code Division Multiple Access (CDMA) cellular wireless communication systems.
2. Related Art
Cellular wireless communication systems are well known. In a cellular wireless communication system a plurality of base stations provide wireless coverage within respective cells that includes respective pluralities of sectors. In aggregation, the base stations work together to support wireless communications within a service area. The base stations are coupled via at least one Base Station Controller (BSC) to a mobile switching center (MSC). The MSC couples to the Public Switched Telephone Network (PSTN) and services voice traffic for all serviced mobile terminals. For the service of packetized communication, e.g., between mobile terminals and the Internet, a packet data link between the BSC and one or more packet data networks is provided.
Various cellular operating standards exist. These cellular standards include, for example, the Advanced Mobile Phone System (AMPS) standards, the Narrowband Advanced Mobile Phone Service (NAMPS) standards, the Global Standards for Mobile Communications (GSM) standards, the Code Division Multiple Access (CDMA) standards, and the Time Division Multiple Access (TDMA) standards among others. These cellular standards are continually improved and updated to service ever increasing cellular wireless communication system demands. Currently, various CDMA systems are deployed, e.g., IS-95A and IS-95B, and additional CDMA systems are planned for deployment, e.g., 1xRTT, 1xEV-DO, etc.
CDMA systems are direct sequence spread spectrum systems in which a plurality of spread-spectrum signals are transmitted simultaneously in the same frequency band, both on forward links from base stations to mobile terminals and on reverse links from the mobile terminals to the base stations. Each intended mobile terminal is assigned at least one distinct Walsh code that identifies the signals sent to, or received from mobile terminal. Of a plurality of forward link spread-spectrum signals intended for a respective plurality of mobile terminals, each is covered by a respective Walsh code. For example, forward link signals intended for a first mobile terminal are covered by a first Walsh code, g1(t), and forward link signals intended for a second mobile terminal are covered by a second Walsh code, g2(t), etc. Each mobile terminal receives a composite spread spectrum signals within the shared frequency band at its antenna, such composite signal including energy intended for all serviced mobile terminals. However, after despreading the received signal with its assigned Walsh code, the receiver of the mobile terminal outputs all the energy of its intended signal but only a small fraction of the energy of signals intended for other mobile terminals. Thus, the signals intended for the mobile terminal may be extracted from the received composite signal by using the assigned Walsh code.
CDMA capacity is interference limited. The number of mobile terminals that may be supported within a particular area is determined by the total interference power that all of the mobile terminals, taken as a whole, generate. The number of mobile terminals that may be supported by each base station is limited because the base station can provide only a maximum power output within its respective cell or sectors that must be divided among the mobile terminals operating within the respective cell or sectors. Thus, as the number of mobile terminals operating within a cell increases, the additional amount of power available for new mobile terminals decreases until a minimal level is reached and a maximum number of users have been reached for the cell or sector. Dividing the power in such a fashion sometimes results in dropped calls due to the signal strength going below a required threshold.
To minimize the dropped call probability and to improve call quality, a mobile terminal typically receives forward link signals from more than one base station and/or from more than one sector of a particular base station, particularly when the mobile terminal is moving from sector to cell or sector to cell. A “rake receiver” of the mobile terminal receives and decodes a plurality of forward link signals and thus can receive and decode these multiple forward link signals. An operation during which a mobile terminal is moving between cells and sectors is commonly referred to as “hand-off.” Hand-off between base stations is generally referred to as “soft hand-off” while hand-off between sectors of a single base station is generally referred to as “softer hand-off.” Mobile terminals continually measure the strength of pilot signals received from cells and/or sectors and report these measurements to the wireless network via a servicing base station. Based upon the reported strengths of these pilot signals, the wireless network then initiates, services, and terminates forward link signals in respective cells/sectors based upon the measurements.
Note however, that a mobile terminal that is stationary will typically also receive forward link transmissions from a plurality of cells and/or sectors even though in some cases a single forward link will service a corresponding mobile terminal. Multiple forward link transmissions to a single mobile terminal may be established even when the mobile terminal initially receives service. During call setup, the wireless network infrastructure typically determines a sector that initially services the mobile terminal. Then, resources are allocated within the base station that services a corresponding forward link and forward link transmissions are initiated within the sector (or cell).
An initially servicing forward link is typically the strongest forward link and is referred to as the “primary” forward link. Weaker forward links that may be enabled during hand-off are typically referred to as “secondary” forward links. When multiple forward links are operational, a serviced mobile terminal obeys directives from the primary forward link, e.g., reverse link power control directives.
In newer CDMA systems, improved Coder DECoder (CODEC) technologies allow voice communications to be supported at lower data rates. Further, improvements in physical layer modulation schemes, Forward Error Coding, and other operations also support lower data rate operations. Because of these technological increases, a greater number of users may be supported within each cell/sector before CDMA interference limitations are reached. For example, in systems using Enhanced Variable Rate CODEC (EVRC) operations, on average, up to 24 users per sector may be supported on each carrier. Further, in 1xRTT systems, on average, up to 30 users per sector may be supported on each carrier.
In a typical IS-95A or IS-95B system, 64 Walsh codes are available for use but at least three Walsh codes are dedicated for use with overhead channels. In IS-2000 systems either 64 or 128 Walsh codes are available, depending upon the implementation, with some of these also dedicated to overhead channels. During operation in which an average of 24 users is supported and with which each user is in hand-off with in average of 2.5 sectors a total of 64 Walsh codes would be required. This operational example would fully deplete the available Walsh codes if 64 Walsh codes were available. When all Walsh codes are used for servicing calls for a set of mobile terminals, new call setup and new hand-offs are blocked.
Thus, there exits a need in the art for a cellular wireless communication system having improved performance during hand-off and call setup.